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On Mon, Oct 07, 2002 at 06:33:39PM -0400, Kent Borg wrote: > I can imagine that a fast, 4-CPU machine, with wicked fast disks might > have use for going a higher, but not as high as, say, 1 GB. Anyone > have a concrete scenario where more swap is useful? I run multiple jobs on multi-gigabyte data sets. I use over a gigabyte of swap in two ways on this load: 1. The working set of each job is about half its total memory consumption, so I can run twice as many jobs by letting it swap out the stuff not in each job's current working set 2. I run some interactive environments that I want to be long-lived while I run several shorter jobs (same memory consumption though), so I'm happy for my interactive jobs to get swapped to disk while the shorter ones run, after which I can return to the original jobs. I've filled up 4GB of swap multiple times doing the above. Owing to the tripped circuit breaker over the weekend I'm not yet back to the point of stressing the machines (maybe later tonight), but here's the current "normal" state for one: total used free shared buffers cached Mem: 3992852 3987344 5508 0 30008 2094152 -/+ buffers/cache: 1863184 2129668 Swap: 2096472 1234100 862372 (No, the 2G free on the +buffer/cache line really isn't free, since it includes some large memory-mapped files which are in the working set. Their pages are clean, but I need them in ram. There's probably only a couple hundred meg really free, which is why 1.2GB are swapped.) I don't like to use more than a couple gig of swap, because it takes a long time for it to get those pages to disk and back. It's better than having jobs killed, though. But, to answer earlier questions, I do find it useful to hang out around using 2GB swap, and the machine works fine doing that. > I guess it boils down to this: bandwidth to disk is a bottleneck, > until disks (and controllers and bus bandwidth) are only so fast and > until they get faster, 256 MB is a lot of swap. More won't hurt, and > with modern big disks is no burden so why not, but I see no benefit. Actually, it can hurt -- the 2.4 kernel sometimes tends to swap things out just 'cause it feels like it. Without any swap space, it would leave the pages in memory, and be much faster to access when you want them. I don't think it's a good idea to run without swap space, but it's not strictly a one-sided issue. BTW, IMHO the best reason to run with swap is because nearly everyone else does, so the software gets the most testing in that situation. Case in point: the 2.4 kernels (starting earlier than you'd want to use them) have an OOM bug which is triggered if you're running without swap. The kernel kills off your processes, claiming it's out of memory, even when most of memory is only being held by the kernel as a disk buffer and should be available. If you have even a small amount of swap, this bug is hidden. [Wasn't fixed by 2.4.18, I haven't looked since then.] --grg
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